1. Field of the Invention
Embodiments of the present invention relate to a transistor, and more particularly, to a thin film transistor (TFT), a method for fabricating the same, and a method of fabricating a liquid crystal display device having the same. Although embodiments of the invention are suitable for a wide scope of applications, it is particularly suitable for forming a channel layer of the TFT through an inkjet process.
2. Description of the Related Art
Generally, an active matrix liquid crystal display (active matrix LCD) that is widely used in a variety of image displays uses a thin film transistor (TFT) as a switching element. A semiconductor layer of the thin film transistor is formed of amorphous silicon. The use of the amorphous silicon is advantageous in fabricating a small-sized TFT LCD. However, since the amorphous silicon has low mobility, it is difficult to use amorphous silicon in fabricating TFTs of a large-sized TFT LCD.
Research for using a polysilicon layer having superior mobility as the semiconductor layer in a TFT has been active. Since polysilicon can be easily used in fabricating TFTs of the large-sized TFT LCD and a drive integrated circuit (IC) can be formed on the substrate on which the TFTs are arrayed, integration can be improved and the fabrication costs can be reduced. Methods of forming the polysilicon layer include directly depositing the polysilicon and crystallizing the polysilicon after amorphous silicon is deposited. Typically, the latter is widely used. That is, after the amorphous silicon layer is formed on a substrate and a crystallizing process is performed to change the amorphous silicon layer into a polysilicon layer.
A polysilicon TFT includes a gate electrode and a source/drain electrode, which are insulated from each other by insulation layers to independently operate. The insulation layer is usually formed of an inorganic insulation material, such as silicon nitride (SiNx) or silicon oxide (SiOx) that has excellent manipulation property and excellent adhesion to metal.
FIG. 1 is a cross-sectional view of a TFT fabricated in accordance with the related art fabrication method. As shown in FIG. 1, a TFT is formed by forming a buffer layer 2 functioning as an insulation layer on a substrate 10 and subsequently forming an amorphous silicon (a-Si) layer on the buffer layer 2. A plasma-enhanced CVD process using SiH4 gas, a low-pressure CVD process, or a sputtering process at a temperature of about 300-400° C. is used to deposit an organic insulation material, such as SiNx or SiOx. After the amorphous silicon layer is formed on the buffer layer 2, an annealing process using an excimer laser is performed to crystallize the amorphous silicon layer into a polysilicon layer, after which the polysilicon layer is patterned into a channel layer 4. Subsequently, the organic insulation material, such as SiNx or SiOx, is deposited over the substrate 10 to cover the channel layer 3, thereby forming a gate insulation layer 5.
Next, a conductive material, such as aluminum (Al) or an Al alloy, is deposited over the gate insulation layer 5 and is patterned through a photolithography process, thereby forming a gate electrode 1 on the channel layer 4. Subsequently, N-type impurities are doped using the gate electrode 1 as a mask, thereby forming an ohmic contact layer 6 on the channel layer 4. At this point, an ion doped region is a region where source/drain electrodes 9a and 9b are formed and a portion of the channel layer 4 under the gate electrode 1 where the impurities are not doped by becomes a channel region.
Next, an organic insulation material, such as SiNx or SiOx is deposited over the substrate 10 on which the gate electrode 1 is formed, thereby forming an inter-insulation layer 7. The deposition of the organic material for the inter-insulation layer 7 is performed by a method identical to that for forming the gate insulation layer 5. Subsequently, contact holes are formed by etching portions of the inter-insulation layer 7 and the gate insulation layer 5.
A metal layer is formed on the substrate 10 on which the contact holes are formed and is etched to form the source/drain electrodes 9a and 9b, thereby completing the polysilicon TFT. However, according to the above-described related art TFT fabrication method, since the masking process is necessary, the fabrication process is complicated. Especially, since the channel layer and the ohmic contact layer formed through the ion doping process are formed through independent processes, the fabrication process is further complicated and the fabrication costs are increase.